1. Field of the Invention
The present invention relates to exposure equipment for use in manufacturing semiconductor devices, and more particularly, to an apparatus for protecting reference marks used in the exposure equipment.
2. Description of the Related Art
Generally, the process of manufacturing semiconductor devices involves a wide variety of steps including cleaning, diffusion, photolithography, etching, ion implantation, and the like. These steps are repeatedly performed on a semiconductor wafer in the mass-production of well-known commercial semiconductor devices.
Of the above-stated steps, photolithography is used for forming patterns on a semiconductor wafer. In this process, the image of the pattern of a reticle is projected on the wafer surface using a photoresist. The photolithography process generally involves several steps, that is, photoresist coating, alignment, exposure, development, and inspection. The photoresist is applied by a spin coater, a stepper is used for the alignment and exposure steps, and the inspection step is conducted using inspection equipment.
Successful completion of the alignment/exposure steps using the stepper depends significantly on the alignment of a reticle and a wafer. To effect a correct alignment, a key pattern is formed on the surfaces of the reticle and wafer. For example, the key pattern on the reticle is aligned with a fiducial or fixed reference mark that is formed on a member mounted on a stage of the stepper.
FIG. 1 shows one part of a conventional stepper for driving a stage 10 on which wafers are placed. The stepper comprises an X-motor 12 and a Y-motor 16 which are oriented along an X-axis and a Y-axis, respectively. These motors 12,16 drive the stage 10 through screws 14 and 18, respectively. The location and alignment of the stage 10 is controlled by the screws 14 and 18.
A lamp check sensor 20, a light intensity sensor 22, and a reference mark assembly 24 are located on respective comers of the stage 10. The lamp check sensor 20 senses (monitors) the state of the light radiated onto the stage 10 and the uniformity of its intensity to thereby check the efficiency of a lamp or light source. The light intensity sensor 22 detects the intensity of light transmitted through a projection reduction lens (not shown) having a reticle (also not shown) mounted thereon. The reference mark assembly 24 is used for reticle alignment, and as shown in FIG. 2, a reference mark member 26 is mounted on the top surface of a base 28. The base 28 is fixed to the stage 10 by bolts 30.
The conventional reference mark member 26 is exposed at the top surface of the base 28, and can be contaminated by particles or impurities while the wafer stage is being checked or cleaned, and the stepper is operating.
As shown in FIG. 3, the reference mark member 26 has a pattern comprising a plurality of reference mark sites 32 which are used for the alignment of the reticle. The mark sites 32 are on the order of a micrometer. FIG. 4A is a representation of signals optically detected from a mark site 32, and the signal from such a mark site 32 is clearly distinguished from those from other regions.
Again, referring to FIG. 3, the reference mark member 26 can be contaminated by a contaminant 34, such as a particle or an impurity. Therefore, when the alignment task is conducted, optically-detected signals from the contaminant 34 are produced along with those from the mark sites 32 as illustrated in FIG. 4B. In a seriously contaminated reference mark member 26 it is difficult to distinguish the mark sites 32 from the contaminated areas 34. Accordingly, a reticle alignment error often occurs in the stepper.
In fact, the conventional reference mark member is so easily contaminated that problems such as reticle alignment error, stepper mis-operation, and exposure defects occur, thus creating problems adversely affecting the reliability of the stepper.